Insights into Enzyme Catalysis and Thyroid Hormone Regulation of Cerebral Ketimine Reductase/μ-Crystallin Under Physiological Conditions

Neurochem Res. 2015 Jun;40(6):1252-66. doi: 10.1007/s11064-015-1590-5. Epub 2015 May 1.

Abstract

Mammalian ketimine reductase is identical to μ-crystallin (CRYM)-a protein that is also an important thyroid hormone binding protein. This dual functionality implies a role for thyroid hormones in ketimine reductase regulation and also a reciprocal role for enzyme catalysis in thyroid hormone bioavailability. In this research we demonstrate potent sub-nanomolar inhibition of enzyme catalysis at neutral pH by the thyroid hormones L-thyroxine and 3,5,3'-triiodothyronine, whereas other thyroid hormone analogues were shown to be far weaker inhibitors. We also investigated (a) enzyme inhibition by the substrate analogues pyrrole-2-carboxylate, 4,5-dibromopyrrole-2-carboxylate and picolinate, and (b) enzyme catalysis at neutral pH of the cyclic ketimines S-(2-aminoethyl)-L-cysteine ketimine (owing to the complex nomenclature trivial names are used for the sulfur-containing cyclic ketimines as per the original authors' descriptions) (AECK), Δ(1)-piperideine-2-carboxylate (P2C), Δ(1)-pyrroline-2-carboxylate (Pyr2C) and Δ(2)-thiazoline-2-carboxylate. Kinetic data obtained at neutral pH suggests that ketimine reductase/CRYM plays a major role as a P2C/Pyr2C reductase and that AECK is not a major substrate at this pH. Thus, ketimine reductase is a key enzyme in the pipecolate pathway, which is the main lysine degradation pathway in the brain. In silico docking of various ligands into the active site of the X-ray structure of the enzyme suggests an unusual catalytic mechanism involving an arginine residue as a proton donor. Given the critical importance of thyroid hormones in brain function this research further expands on our knowledge of the connection between amino acid metabolism and regulation of thyroid hormone levels.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acids / metabolism
  • Brain / enzymology*
  • Catalysis
  • Crystallins / antagonists & inhibitors
  • Crystallins / metabolism*
  • Enzyme Inhibitors / pharmacology
  • Humans
  • Hydrogen-Ion Concentration
  • Imines / pharmacology
  • Kinetics
  • Metabolic Networks and Pathways / drug effects
  • Models, Molecular
  • Molecular Docking Simulation
  • Nitriles / pharmacology
  • Oxidoreductases Acting on CH-NH Group Donors / antagonists & inhibitors
  • Oxidoreductases Acting on CH-NH Group Donors / metabolism*
  • Pipecolic Acids / metabolism
  • Substrate Specificity
  • Thyroid Hormones / physiology*
  • Thyroxine / pharmacology
  • Triiodothyronine / pharmacology

Substances

  • Amino Acids
  • Crystallins
  • Enzyme Inhibitors
  • Imines
  • Nitriles
  • Pipecolic Acids
  • Thyroid Hormones
  • ketimine
  • Triiodothyronine
  • crystallin mu
  • Oxidoreductases Acting on CH-NH Group Donors
  • ketimine reductase
  • Thyroxine